1. Field of the Disclosure
The disclosure relates generally to optical devices and, more particularly, to optical devices configured for single-mode transmissions.
2. Brief Description of Related Technology
Optical networks often utilize single-mode fiber rather than multimode fiber. Optical signals traveling in single-mode fiber generally experience less dispersion and, as a result, single-mode fibers can carry more information over longer distances. Further performance advantages arise from the many ways in which single-mode transmission of light may be manipulated, most of which are not possible (or highly impracticable) with multimode light. For example, single-mode signals are easily amplified without requiring a conversion to electrical signals. Filtering and switching can also be performed according to wavelength and phase. In contrast, signal control inside multimode fiber is very complex, with typical multimode optical fiber often having over 100 modes available for the light energy to occupy. The modes active at any moment are thus largely beyond control.
Notwithstanding the high data transfer rates and other benefits of single-mode transmission, the use of single-mode fiber presents a number of challenges, particularly at connection points involving one or more fibers. For example, FIG. 1 shows a common butt-joint connection in which two single-mode fibers 20, 22 are abutted in a non-permanent connection (i.e., a connection configured to support one or more coupling or de-coupling steps). To facilitate the alignment and abutment of the mating fibers 20, 22, the connection typically involves the physical arrangement of a plug 24 and corresponding receptacle 26. The challenge is to align the fibers sufficiently to achieve low-loss coupling. Unfortunately, sub-micron tolerances are often required. Moreover, an initial alignment may be difficult to maintain when the connection is submitted to vibrations or thermal changes.
Complicating matters further, dirt or other particle contamination present at or near the center of the connection can cause catastrophic failure, especially in high-power applications. The single-mode fibers 20, 22 typically have a very small core 28 of about four to eight microns in diameter, which can easily be permanently scratched by dirt or other particles during coupling and decoupling operations. Even if catastrophic failure is avoided, dirt can permanently degrade the optical performance of a standard single-mode fiber connection. For these reasons, the standard single-mode connection shown in FIG. 1 is typically not suitable for use in a number of environments, e.g., where dirt is present.
In the event of a scratch, a single-mode connection usually needs to be replaced. Connection replacement is often performed in the field, typically by a highly trained technician. In such cases, physical access to the connectors and abutting fibers can be problematic. For these reasons, single-mode connectors are typically not used when the connector cannot be easily accessed, such as inside a liquid tank.
Even when dirt is absent, standard single-mode fiber connections are often unsuitable for high-power applications. The small output cross-section of the core 28 is susceptible to surface damage if the optical signal intensity exceeds a relatively low value.
These alignment and other challenges presented by the traditional butt-joint connections have been addressed in the past via the insertion of an intermediary optical element. Some intermediary optical elements have been physically separated from the fiber, while others have been integrated within the fiber. In either case, the optical cross-section of the connection is often expanded to maximize the transmission through the connection. The wider cross-section relaxes the transverse alignment accuracy requirements and also reduces the susceptibility to contamination during subsequent coupling and decoupling steps. The connectors used in this type of connection are often referred to as expanded beam single-mode connectors, or single-mode tapered connectors. See, for example, Carlsen U.S. Pat. No. 4,421,383, entitled “Optical Fiber Connectors.” In a typical expanded beam single-mode connector, the mode size is expanded from its nominal single-mode value to a larger size (e.g., a larger diameter). In the corresponding, or mating, connector (i.e., the second half of the connection), the beam diameter is reduced in size as it propagates down a corresponding taper toward the output fiber.
Unfortunately, tapered connectors, such as modified fibers, lenses (or micro-collimators), and integrated, tapered waveguides, have generally been hampered with shortcomings. For example, fibers have been tapered using fusion, up-tapering of a fiber preform, and dopant diffusion via heat treatment. Unfortunately, none of these techniques, has been shown to generate sufficiently large cross sections. See, for example, the following papers: Furuya, et al., “Low loss splicing of single-mode fibers by tapered-butt-joint method,” Trans. IECE Jpn., vol. E61, p. 957 (1978); Amitay, “Optical fiber tapers—A novel approach to self-aligned beam expansion and single-mode hardware,” J. Lightwave Technol., vol. LT-5, p. 70 (1987); Shigihara, et al., “Modal field transforming fiber between dissimilar waveguides,” J. Appl. Phys., vol. 60, p. 4293 (1986); Ohtera, et al., “Numerical analysis of eigenmodes and splice losses of thermally diffused expanded core fibers,” J. Lightwave Technol., vol. 17, pp. 2675-2682 (1999). In these and other cases, the tapering ratio (i.e., the output diameter relative to the input diameter) has been limited to undesirably small values.
In connectors having a lens or micro-collimator, the light beam emitted from a fiber end is expanded and collimated by a lens (e.g., a GRIN lens) and directed at the other half of the connection via free-space propagation. A corresponding connector element of the second-half of the connection then receives the collimated, larger diameter beam after free-space propagation, and focuses it onto the end of another fiber. Because the beam is of relatively large diameter when it is transferred from one connector element to the other connector element, the lateral alignment accuracy requirement is reduced. Examples include the STRATOS Lightwave™ connectors commercially available from Glenair, Inc. (www.glenair.com, Glendale, Calif.) and those described in Ukrainczyk U.S. Pat. No. 6,632,025, entitled “High power expanded beam connector and methods for using and making the high power expanded beam connector.”
Many commercial single-mode expanded beam connectors are based on the free-space lens design. Unfortunately, these expanded beam connectors are often very sensitive to vibration and dust, making them unsuitable for operation in demanding environments. Moreover, the lenses are often undesirably bulky, and generally incompatible with high-density fiber connectivity. Still further, reductions in lateral alignment requirements may come at the price of increased angular alignment sensitivity.
Integrated tapered waveguides are often used to convert optical mode sizes to couple optical devices of different cross-sectional dimensions. Despite common use in optical mode size conversions, integrated tapered waveguides generally present efficiency problems. To achieve an efficient power transfer, the guided mode should evolve through the integrated taper in adiabatic fashion. In other words, the taper is designed in theory to let the optical signal propagate under radiation-loss-free and mode-conversion-free conditions. Integrated and other waveguides have been difficult to taper for adiabatic transmission through the connection.
A number of methods of increasing the coupling efficiency of tapered waveguides are described in the literature. The methods are broadly classified into three categories, cross-sectional dimension tapering, index tapering, or a combination of both, as discussed in the following papers:    (i) Mitomi, et al., “Design of a single-mode tapered waveguide for low-loss chip-to-fiber coupling,” IEEE J. Quantum Electron., vol. 30, pp. 1787-1793 (1994);    (ii) Suchoski, Jr., et al., “Constant-width variable index transition for efficient Ti—LiNbO3 waveguide-fiber coupling,” J. Lightwave Technol., vol. 5, pp. 1246-1251 (1987);    (iii) Yanagawa, et al., “Index-and dimensional taper and its application to photonic devices,” J. Lightwave Technol., vol. 10, pp. 587-591 (1992); and,    (iv) Yamaguchi, et al., “Low-loss spot-size transformer by dual tapered waveguides (DTW-SST),” J. Lightwave Technol., vol. 8, pp. 587-593 (1990).
Theoretically, completely adiabatic tapered waveguides are realized by simultaneously altering the refractive index and the cross-section of the waveguide. However, this result is very difficult to achieve with the lithographic tools that are commonly used when manufacturing the planar circuitry of integrated devices. Lithographic and other fabrication tools are generally suited for tapering in a single plane. The production of such integrated devices is also typically undesirably complicated via the use of multiple materials, substrates or other components arranged in complex structures.